1. Field of the Invention
Embodiments of the present invention relate generally to photographic systems, and more specifically to systems and methods for improved color balance in digital photography.
2. Description of the Related Art
A typical digital camera generates a digital photograph by focusing an optical image of a scene onto an image sensor, which samples the optical image to generate an electronic representation of the scene. The electronic representation is then processed and stored as the digital photograph. The image sensor is configured to generate a two-dimensional array of color pixel values from the optical image, typically including an independent intensity value for standard red, green, and blue wavelengths. The digital photograph is commonly viewed by a human, who reasonably expects the digital photograph to represent the scene as if observed directly. To generate digital photographs having a natural appearance, digital cameras attempt to mimic certain aspects of human visual perception.
One aspect of human visual perception that digital cameras mimic is dynamic adjustment to scene intensity. An iris within the human eye closes to admit less light and opens to admit more light, allowing the human eye to adjust to different levels of light intensity in a scene. Digital cameras dynamically adjust to scene intensity by selecting a shutter speed, sampling sensitivity (“ISO” index of sensitivity), and lens aperture to yield a good exposure level when generating a digital photograph. A good exposure generally preserves subject detail within the digital photograph. Modern digital cameras are typically able to achieve good exposure for scenes with sufficient ambient lighting.
Another aspect of human visual perception that digital cameras mimic is color normalization, which causes a white object to be perceived as being white, even under arbitrarily colored ambient illumination. Color normalization allows a given object to be perceived as having the same color over a wide range of scene illumination color and therefore average scene color, also referred to as white balance. For example, a white object will be perceived as being white whether illuminated by red-dominant incandescent lamps or blue-dominant afternoon shade light. A digital camera needs to compensate for scene white balance to properly depict the true color of an object, independent of illumination color. For example, a white object illuminated by incandescent lamps, which inherently produce orange-tinted light, will be directly observed as being white. However, a digital photograph of the same white object will appear orange without compensation for the orange white balance imparted by the incandescent lamps. To achieve proper white balance for a given scene, a digital camera conventionally calculates gain values for red, green, and blue channels and multiples each component of each pixel within a resulting digital photograph by an appropriate channel gain value. By compensating for scene white balance in this way, an object will be recorded within a corresponding digital photograph as having color that is consistent with a white illumination source, regardless of the actual white balance of the scene. In a candle-lit scene, which is substantially red in color, the digital camera may reduce red gain, while increasing blue gain. In the case of afternoon shade illumination, which is substantially blue in color, the digital camera may reduce blue gain and increase red gain.
In scenarios where a scene has sufficient ambient lighting, a typical digital camera is able to generate a digital photograph with good exposure and proper white balance. One technique for implementing white balance compensation makes a “gray world” assumption, which states that an average image color should naturally be gray (attenuated white). This assumption is generally consistent with how humans dynamically adapt to perceive color.
In certain common scenarios, ambient lighting within a scene is not sufficient to produce a properly exposed digital photograph of the scene or certain subject matter within the scene. In one example scenario, a photographer may wish to photograph a subject at night in a setting that is inadequately illuminated by incandescent or fluorescent lamps. A photographic strobe, such as a light-emitting diode (LED) or Xenon strobe, is conventionally used to beneficially illuminate the subject and achieve a desired exposure. However, the color of the strobe frequently does not match that of ambient illumination, creating a discordant appearance between objects illuminated primarily by the strobe and other objects illuminated primarily by ambient lighting.
For example, if ambient illumination is provided by incandescent lamps having a substantially orange color and strobe illumination is provided by an LED having a substantially white color, then a set of gain values for red, green, and blue that provides proper white balance for ambient illumination will result in an unnatural blue tint on objects primarily illuminated by the strobe. Alternatively, a set of gain values that provides proper white balance for the LED will result in an overly orange appearance for objects primarily illuminated by ambient incandescent light. A photograph taken with the LED strobe in this scenario will either have properly colored regions that are primarily illuminated by the strobe and improperly orange regions that are primarily illuminated by ambient light, or improperly blue-tinted regions that are primarily illuminated by the strobe and properly colored regions that are primarily illuminated by ambient light. This photograph will conventionally include regions that are unavoidably discordant in color because the white balance of the strobe is different than that of the ambient illumination.
One approach to achieving relatively consistent white balance in strobe photography is to flood a given scene with illumination from a high-powered strobe or multiple high-powered strobes, thereby overpowering ambient illumination sources and forcing illumination in the scene to the same white balance. Flooding does not correct for discordantly colored ambient light sources such as incandescent lamps or candles visible within the scene. With ambient illumination sources of varying color overpowered, a digital camera may generate a digital photograph according to the color of the high-powered strobe and produce an image having very good overall white balance. However, such a solution is impractical in many settings. For example, a high-powered strobe is not conventionally available in small consumer digital cameras or mobile devices that include a digital camera subsystem. Conventional consumer digital cameras have very limited strobe capacity and are incapable of flooding most scenes. Furthermore, flooding a given environment, such as a public restaurant or indoor space, with an intense pulse of strobe illumination may be overly disruptive and socially unacceptable in many common settings. As such, even when a high-powered strobe unit is available, flooding an entire scene may be disallowed. More commonly, a combination of partial strobe illumination and partial ambient illumination is available, leading to discordant white balance within a resulting digital photograph.
As the foregoing illustrates, what is needed in the art is a technique for generating a digital photograph having consistent white balance with partial strobe illumination.